I think a mechanical engineer or a materials
scientist would know the most about important
properties of materials.

I bet that not all submarines are made from all of
the same materials. Like for most things, we
choose materials based on what we want them to do.

I can think of a few things that all submarines
need to do. First, a submarine should be
watertight: it needs to keep water out, and air
in, so that people inside can breathe. Second,
it should be able to withstand high
pressure. As a submarine sinks deeper, the
water around it will push harder on its walls from
all directions. We don't want the hull of the
submarine to buckle or break. From these things,
I would choose a strong, hard material to make
the hull, like a steel alloy.

Third, we don't want the outer hull to rust,
since it is always touching the water. Rust is
made when iron (the main component in steel) and
oxygen react, which happens more easily in water.
Rust is bad for our submarine because it is a
weaker material: it is flaky and breaks apart
easily.

We could make a better submarine by looking for
materials that are better in some important way.
For example, if I found a material that had a
higher bulk modulus than steel (that means it's
stronger against compression), it might be good
for making a hull, and it might let my submarine
explore even deeper in the ocean. Of course, I
would also have to be able to make it watertight
and rust-resistant. I'm sure that there are even
more important properties that a submarine expert
would know about.

People have been making submarines for more
than a hundred years, so by now, I think we
probably have very good materials. If you wanted
to make it better than what we have now, you would
need to have a good idea for how to find a new
material (or maybe how to build a submarine). You
could make a lot of money doing that, too.